As digital-camera and phone-camera
resolution increases, high-resolution
image sensors require more light. Firstgeneration
low-res camera phones provided
barely adequate flash intensity for
taking closeups of friends at parties. Even
second-generation camera-phone flash
is still unsuitable for image sensors with
greater than 3-Mpixel resolution. And
that’s just talking about still photography.
Video requires in-phone camera lighting
to provide movie/torch mode.
How are the phone-camera OEMs
to achieve that illumination capacity
without seriously impacting talk time?
CAP-XX is working with a number of
U.S. semiconductor companies that
are designing LED-flash ICs that take advantage of supercapacitors’ deep
charge-storage capabilities to drive highbrightness
white LEDs, which naturally
have high peak current demands.
So far, charge-pump-based white LED
flash driver designs have limited current
of up to about 800 mA. Alternative
designs that rely on inductive-based stepup
converters have been limited to 1 to
1.5 A. Higher currents would be possible,
but throwing a large current demand
directly onto the cell phone’s lithium battery
would pull the battery voltage down
because of the battery’s equivalent series
resistance (ESR), possibly enough to
result in a CPU reset in the phone.
Nonetheless, higher currents are needed.
High-intensity LEDs require large
currents. Seoul Semi, Philips Lighting,
and other companies have announced
high-intensity LEDs requiring more
than 1 A. That implies a high forward
voltage drop, because LED VF is proportional
to ID. Obviously, this is a job for
supercapacitors, particularly the smaller
board-mounted devices with values ranging
from 30 mF up to 2 F that CAP-XX
specializes in.
These supercaps have breakdown voltages
around 2.75 V per cell, so they must
be connected in series, with appropriate
cell balancing resistors. Using them
requires some extra circuit complexity
because their low ESR (on the order of
50 to 100 mO) requires some means to
manage charge current.
According to Pierre Mars, CAP-XX’s
vice president of applications engineering,
Advanced Analogic Technologies
was the only semiconductor company as
of June to have introduced a driver chip
for use with supercapacitors in white
LED cell-phone flash applications. The
AAT1282 combines a high-frequency
boost converter with fixed input current
limiting, dual-output regulated current
sinks, and I2C control.
Those output current sinks each
provide a regulated 1 A. The chip’s two
LED current sources share the output
current equally. They can be connected
together to apply a full 2-A output into a
single white LED, or each diode can be
connected to its corresponding current
source, and the driver can provide two
independently controlled 1-A outputs.
The battery-current limiter in the
step-up converter that charges the
supercapacitor prevents large bus voltage
drops that might cause the phone to
reset. Operating at a 2-MHz switching
frequency, the boost converter needs
only a 1-µH inductor and a 2.2-µF output
capacitor.
The serial digital input can be used to
enable, disable, and set the movie-mode
(torch) current for each flash LED across
16 levels and for other control functions.
More basically, a single external resistor
sets the maximum flash and movie-mode
current. (The ratio between flash current
and movie-mode current is preset at
7.3:1, but it can be adjusted.)
A flash-enable pin initiates the flash
operation. It also can be used to control
a timer for shutting off the flash after
a user-programmed delay. A load-disconnect
function isolates the load from
the power source while in the OFF or
disabled state, limiting shutdown current
to less than1.0 µA. All this comes in a
14-pin, 3- by 3-mm package. The device
costs $1.75 in 1000-unit quantities.
See associated figure
DON TUITUE
ANALOGIC TECH
www.analogictech.com
CAP-XX
www.cap-xx.com